Vacuum tube device



Jan. 5,- l932.. l. SUTHERLIN 3,839,904

VACUUM TUBE DEVICE Filed Aug. 14,y 1926 Muual Candi/c fange F lame/1f Carre/lf INVENTOR Lee u/er//h l- ATTRNEY Patented Jan. 5, 1932 UNITED STATES PATENT OFFICE LEE SUTHERLIN, OF EAST PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMEANY, A CORPORATION OF Il?ENNSYIN'ANIAl VACUUM TUBE DEVICE Application iled August 14, 1926. Serial No. 129,120.

My invention relates to three-electrode discharge tubes and among its principal objects is the provision of an improved radio-receiving tube, or the like, in which the electrode elements are so constructed and arranged that the output of the tube decreases in quantity or quality, or both, if it is at-tempted to increase the filament current above the normal operating. value.

The ordinary radio-receiving tube, as made heretofore, is very often destroyed, or seriously damaged, by reason of the natural inclination ofthe person using the same to turn the filament resistor until the sound or signal reproduction in the loud speaker, or the like,

is of maximum strength or of optimum quali-v ty. The radio-receiving tubes of the prior art have, in this respect, directly invited their destruction, inasmuch as the characteristics 2O of such tubes were such that the output became greater and better as the filament current was increased. Very often, the inexperienced operator, and, sometimes, even the experienced one who had no suitable meter available for measuring the filament current, kept on increasing the current until the lilament finally burned out.

A radio-receiving tube made according to my invention is so designed that it offers a natural check against the tendency of the operator to increase the filament current above the normal value for which the tube is designed. This is achievedby so dimensioning and shaping the electrode elements in the tube that the performance characteristics of the tube when operated with normal filament current are as good as, or even better than, those of the prior-art tubes but, upon increasing the heating current above the normal value, no increase in out-put is obtained. As a matter of fact, my improved tube may be so designed that an increase of the filament current above the normal operating value will cause a decrease in the magnitude of the output, thus directly inducing the operator to decrease the heating current through the filament, thereby preserving the life and usefulness of the tube.

In the accompanying drawings,

tFig. 1 is an elevational view of a radio receiving tube embodying my invention, parts of the envelope and electrode structure being broken away,

Fig. 2 is an enlarged sectional view diagrammatically illustrating the arrangement of the cathode, grid, and plate of the tube shown in Fig. l,

Fig. 3 is a curve diagram showing the characteristifs of the 'improved tube made according to my invention, in comparison with prior-art tubes, and

Fig. 4 is a view similar to Fig. 2 illustrating a modification of my invention.

As explained above, my invention has particular reference to radio-receivin tubes since it is in the latter eld that t e need therefor is greatest. However, the broad principles underlying my invention are not neecssarily limited to radio receiving tubes.

For purposes of more clearly explaining my invention, I shall describe its application to a radio receiving tube, such as shown in Fig. l, in which a glass envelope l has a reentrant press 2, on which is mounted an electrode assembly 3 comprising a straight line filamentary cathode 4 surrounded by a cylindrical grid helix 5 which is, in turn, surrounded by a concentric cylindrical anode or plate 6.

The upper and lower ends of the cathode filament ll, at the points where it approm'- mately emerges from the plate cylinder 6, has two connector tips 7 welded or secured thereto to permit the mounting of the filament on a pair of carrier rods 9 which are fused into the press 2 and extend to above and below the cathode filament, respectively. The grid 5 and the plate 6 are similarly held in place by carrier rods 1l and 12, held by the press.

The operating characteristics of a priorart tube of a type designed to be heated from ordinary dry cells, or similar low-power current source, is represented by the full-line curve 15, in Fig. 3. The abscissas of the 95 curve give the filament currents and the ordinates give the' mutual conductance of the tube, that is the ratio of the am lication factor to the plate impedance. I the lilament current is increased, the prior-art tube 100 under consideration will improve in per-4 formance and increase in output, since the mutual conductance curve keeps on increasing for values of filament current above the normal filament current, which is, in this case, approximately 250 milliamperes. i

My improved tube distinguishes from that of the prior art in that the electrode elements are so designed and related to each other that the branch of the characteristic curve above the normal filament-operating point is flattened or even drooping. In this case, an increase in the filament current above the normal value does not increase the mutual conductance of the tube but, von the contrary, may even cause a decrease of the same, as indicated b `the broken-line curve 16 which represents the corresponding characteristic of a specimen of my improved tubes.

It is clear that, when operating an improved tube, having a characteristic such as shown in the broken line curve 16, there will be no incentive for the operator to increase the filament current above the normal value, since it will not improve the volume or uality of the output but, on the contrary, w ere the tube has a characteristic which slopes downward at values above the normal fila? ment current, the tube output will decrease at the larger lament currents. Within the normal operating range, the improved tube has, thus, all the advantages of the prior-art tubes, but it has, in addition, the very material advantage that it is less liable to be damaged or burnt out in' the course of handling by inexperienced operators.

There are a variety of ways in which tubes having fiat upper branches of the mutual characteristic curve may be obtained. In the tube shown in Fig. l, this result has been secured by suitably dimensioning the length of the grid with respect to the length of the filament, as shown more in detail in Fig. 2 which illustrates the relative proportions of the length of the filament 4, grid 5 and plate 6. In'this construction, the active length of the filament 4, between the terminal tips 7, is shown to be approximately equal to the length of the plate 6, while the length of the grid 5 is somewhat less than that of the filament. Filament 4 thus has two end portions 21 adjacent to the terminal tips 7 which are not surrounded by the grid 5, and, accordingly, electrons that would be emitted from these end portions of the filament would be less subject to control action from the grid than the portions lying within the grid cylinder. In other words, the amplification factor for the two end portions of the discharge tube at the ends of the filament outside the grid cylinder is very small, as compared with the amplification factor of the central portion of the tube in which the cathodg filament is surrounded by the grid cyl- 1n er.

The end portions 21 of the filament, adjacent to the connecting tips 7 will not be as hot as the central lmain portion of the filament disposed within the grid cylinder 5, on account of cooling effect of the relatively large filament tips 7. By properly proportioning the shape and size of the filament tips 7 or, in general, the members which support the filament ends, it is possible to keep the temperature of the filament ends 21, when heated with the normal filament current, sufficiently low to substantially restrict the electron emission to the central portion of the filament which lies within the grid cylinder,

that is, to the portion of the tube in which' the grid exercises this full control action and has a high amplification factor.

Under the foregoing conditions, the tube will, at the normal operating point, have an amplification factor and plate impedance which is approximately equal to that of a similar prior-art tube, thus giving a mutual conductance of approximately the same value as the prior-art devices.

The advantages of my improved tubev appears clearly when it is considered what happens when the filament current is increased above its normal value. In the tubes of the prior art, when the filament current was increased and the cathode emission thereby enlarged, the impedance of the tube rapidly fell off without a correspondingly large decrease in the amplification factor, thus giving an increase in the mutual conductance with increasing filament current. In distinction therefrom,.an increase of the filament current in my improved tube, such as shown in Figs. 1 and 2,-heats the formerly cool end portions 21 of the filament 4 to an electron-emitting temperature, and the electrons from this-portion of the tube begin to exercise a very material influence upon the tube characteristics, inasmuch as there are added two discharge portions which are substantially-outside the control action of the grid, to that of the center which is within the full control action of the grid, thus decreasing the effective resultant amplification factor of the tube. Under these conditions, the decrease of the plate impedance which takes place upon an increase of the filament emission is accompanied by a material decrease of the amplication factor, and, by suitably proportioning the length of the filament ends which are outside the grid and the temperature gradient near the filament tips 7, it is possible to so make the relative changes in the plate impedance and amplification factor that the ratio of the two does not increase with the increasing filament current, as was the case in prior-art tubes. As a matter of fact, the proportions and spacing of the tube elements may be so chosen that the decrease in amplification factor may even be greater than the decrease of the plate impedance', thus causing the mutual conductance to fall off when increasing the filament current above normal.

Thus, in my limproved construction, an increase in the filament current is accompanied with additional emission of electrons at the portions of the filament where they may pass to the plate with but little control effect from the grid. `.As a result thereof, the amplification factor of the tube rapidly decreases. The more the filament current is increased, the more nearly the filament temperature at the ends reaches the value at which it emits electrons in amounts comparative to the central portions ofthe filament, and, consequently, the more the amplification factor of the tube becomes reduced.

By properly dimensioning the electrode elements and the supports thereof, the mutual conductance curve may thus be made flat or drooping. In the preferred construction shown in the drawing, the anode cylinder is made approximately of the same length as the filament, but it may be longer or shorter. The grid should preferably be somewhat shorter than the plate. The vital feature of this construction is the adjustment of the length of the grid with respect to the points of the filament where electron emission substantially ceases at normal operating currents. There is a considerable amount of fiexibility in the design so -that the curve may be fiattened, or made drooping within a relatively wide range.

The'curves in Fig. 3 have been obtained from actual tubes.l 'The prior art tube, the characteristics of lwhich are represented by curve 15, had a plate cylinder 78 long, and 5-f in diameter; a grid of the same length as the plate and g in diameter; and a filament fr long. The improved tube, the characteristics of which are represented by broken-line curve 16, had a plate long and g in diameter; a grid 1% long, in diameter; and a filament 4% long. vWithin the normal operatingrange, the iinproved tube was, in every respect, at least-as good as a tube with the rising mutual-conductance characteristic.

In addition to the advantages of the improved tube in eliminating the tendency of the operators to unduly increase the filament-heating currents, its fiat mutual-conductance characteristics makes it also particularly well adapted for the construction of radio receiving sets, or similar devices, in which the usual filament resistor or cathodetemperature-controlling means is entirely eliminated. For example, an improved tube with the flat-mutual-conductance characteristics may be directly connected to a storage battery, or the like, and the tube will operate equally well throughout the entire range of voltage variations of the batteries from full charge to discharge. By reason of the special characteristics of my new tube, the

variations of the cathode-heating or filamentsupply voltage are so automatically compensated that a variation of the tube output that would tend to result from an Aincreased electron emission incident to increased heating is automatically offset by the accompanying degrease in the amplification factor of the tu e.

As pointed out above, the tube performance depends, not only on the dimensions of the electrode, but also on the details of construction thereof, or, in general, on the elements which effect the dissipation of the heat from the filament, more particularly, from the tips 7 which hold the filament ends. For instance,.the rate of the heat dissipation at the filament ends may be changed, and the character of the tube characteristics thereby affected, by a construction such as shown in Fig. 4, wherein filament tips 22 of larger mass than the tips 7 in Fig. 2 are provided. The enlarged filament tips 22 may also have cooling fins, indicated at 23, for suitably adjusting the rate of heat dissipation at the filament ends.

The modification of the invention shown in Fig. 4 further distinguishes by a modified construction of the grid 24, which has a wider vmesh at the ends than at the central portion, thus producing an additional, decrease of the controlling action of the grid at the portionswhich tend to emit an increasing amount of electrons with increasing filament current. l

In the following claims, the' statement that a cathode is normally heated means that its hottest part is substantially at the highest temperature at which it is capable of operating without reducing, below a reasonable value, the period during which the cathode can properly performits expected emissive function. When the hottest spot of the cathode is at a higher temperature than the foregoing, the cathode is overheated. In these claims also the word emissive means emissive of electrons at a substantial rate. A statement that an electrode is positioned to control an electron flow means that the parts of the electrode are so disposed in space that, when negatively charged relative to the cathode, it will substantially impede the flow of electrons to the anode. Persons skilled in the art can readily tell whether a given electrode does or does not control a given electron ow in accordance with this definition. In these claims, also, the word grid signifies a discontinuous or meshed structure between conducting bars of which are lspaces through which electrons may ow.

Many other modifications will suggest themselves to those skilled in the art, and it is desired that the appended claims shall be construed broadly to cover all constructions falling within the scope and spirit of my invention.

I claim as myinvention:

l. An electron discharge tube comprising an anode, a cathode and a grid surrounding the cathode Within the anode and being shorter at each end than the cathode and anode and having perforations through which electrons pass and which are further apart from each other at the end of the grid than at the middlethereof.

2. A11 electron discharge tube comprising an anode, a cathode and a grid surrounding the cathode Within the anode, said grid having perforations through which electrons pass and Whichl are further apart from each other at the end of the grid than at thev middie thereof. p

In testimony whereof, I have hereunto subscribed my name this 11th day of August,

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